Modulation of steroidal levels by adrenalectomy/castration and inhibition of neurosteroid synthesis enzymes affect sigma1 receptor-mediated behaviour in mice.

The interaction between neurosteroids and sigma1 (σ1) receptors may be of therapeutic interest during physiological or pathological ageing, particularly concerning their neuromodulatory role on cognitive functions. Neurosteroids modulate memory processes through a mechanism involving interactions with GABAA, N-methyl- d-aspartate and/or σ1 receptors. To measure the contribution of endogenous neurosteroid levels to the antiamnesic effects of σ1 agonists, we investigated the effects of inhibitors of key enzymes involved in neurosteroid synthesis, in adrenalectomized/castrated (AdX/CX) mice to avoid the effect of circulating steroids. Trilostane, a 3β-hydroxysteroid-deshydrogenase inhibitor, blocks the pregnenolone to progesterone conversion and leads to a decrease of progesterone. Finasteride, a 5α-reductase inhibitor, blocks the progesterone to 5α-pregnane-3,20-dione conversion and leads to an accumulation of progesterone. The in vivo binding of (+)-[3H]SKF-10 047 to σ1 sites was measured in the mouse hippocampus and cortex. The attenuating effect of the selective σ1 agonist PRE-084 (0.1–3 mg/kg) against dizocilpine (0.15 mg/kg)-induced learning impairment was examined using spontaneous alternation behaviour, step-down passive avoidance and place learning in the elevated plus-maze. The in vivo (+)-[3H]SKF-10 047 binding appeared significantly increased in AdX/CX mice and after trilostane treatment (10 mg/kg twice a day, 7 days), compared with sham-operated animals. The finasteride treatment (25 mg/kg, 7 days) significantly decreased binding levels. The learning deficits induced by dizocilpine were not affected by the treatments. The antiamnesic effect of PRE-084 was facilitated in AdX/CX mice and even more after trilostane treatment, as several parameters for animals treated with both PRE-084 and dizocilpine returned to control values. The PRE-084 effect was blocked after finasteride. These results confirmed that endogenous neurosteroidal levels modulate σ1 receptor-mediated behaviour directly, and revealed that, among neurosteroids, progesterone may be the main modulator of σ1 receptors.